In the application of a spatial light modulator to a projection-type display apparatus or holographic television set, when information is written in the spatial light modulator by using a CRT, the driving pulses of the spatial light modulator are generally applied in synchronism with one frame cycle of the CRT. Such a driving method is reported, for example, in SID Digest (1991) Page 254-256. Timing charts for driving the CRT and spatial light modulator are shown in FIG. 2. In FIG. 2, time-chart (a) shows the driving pulses of the CRT; time-chart (b) shows the luminance intensity of a phosphor on the screen of the CRT; time-chart (c) shows the driving signals of the spatial light modulator; and time-chart (d) shows the intensity of the output light from the spatial light modulator. The spatial light modulator is driven by the driving signals 204 which are applied in synchronism with the driving pulses 201 of the CRT. One driving signal 204 consists of an erasing pulse 202 and a reading period 203. When a phosphor on the screen of the CRT outputs a writing light 205 at a time in the reading time 203 of the driving signal 204, the light is received by a photo-conductive layer of a pixel of the spatial light modulator, the pixel of spatial light modulator turns on and starts up outputting an output light 206. When the erasing pulse 202 is inputted to the spatial light modulator, the spatial light modulator turns off and the intensity of the output light 206 becomes zero. By repeating the above-mentioned actions, an image is observed on the spatial light modulator. This method has the merit that the duty ratio of the output light (total on time of the output light in one frame cycle) can be made larger even when a material having a short life is used as a phosphor, and thereby a bright image can be obtained from the spatial light modulator. Generally, when a large negative voltage is applied to the spatial light modulator, the spatial light modulator is turned on by an electric field generated by the negative voltage even when the writing light is not inputted thereto. However, the driving signals 204 have the wave forms shown in the time-chart (c) in FIG. 2, so they do not apply any large negative voltage to the spatial light modulator. Therefore, the spatial light modulator may not turned on by the electric field and the image contrast ratio of the image is not reduced so much. In addition, it is reported that half tone representation can be obtained by changing the luminance intensity of the phosphor on the CRT.
When the electric field applied on a ferroelectric liquid crystal is very high, ions are generated in the liquid crystal by polarization thereof. The ions are accumulated in the boundary of the ferroelectric liquid crystal and an alignment film. Therefore, a threshold voltage for switching the ferroelectric liquid crystal is varied by the amount of the accumulated ions. When the threshold voltage is changed, deterioration appears in the switching characteristics of the ferroelectric liquid crystal with repetition of the driving. For preventing the appearance of the deterioration of switching characteristics, the wave forms of the driving signal are generally made to be symmetrical with respect to the polarity of voltage. By such measures, the mean value of the displacement of the ion in the ferroelectric liquid crystal is made to be zero. And thereby, tolerance for the deterioration of the ferroelectric liquid crystal can be increased. An example that such driving signals are used for driving the spatial light modulator is described in, for example, Japanese Applied Physics 30 (1991) Page L386-L388. Examples of the driving signals are, for example, shown in FIGS. 3 and 4 of this application, as well. In FIG. 3, time-chart (a) shows the wave form of the driving signal of the spatial light modulator, and time-chart (b) shows the intensity of the output light from the spatial light modulator. As shown in the time-chart (a) in FIG. 3, the driving signals are formed to be repetitions of an erasing pulse 301 and a writing pulse 302, and these two pulses 301 and 302 have substantially the same width and absolute value as the voltage but their polarities are opposite to each other. When such driving signals are applied to the spatial light modulator, the output light is turned off without regard to the existence of the inputted light during the period of erasing pulse 301, and the output light is issued only when the inputted light exists during the period of writing pulse 302. Accordingly, the intensity of the output light 303 responding to the inputted light is shown by the time-chart (b) in FIG. 3. FIG. 4 shows another example of the driving signals. In FIG. 4, time-chart (a) shows the wave form of the driving signals of a spatial light modulator, and time-chart (b) shows the intensity of the output light from the spatial light modulator. As shown in time-chart (a) in FIG. 4, one driving signal consists of an erasing pulse 401, a first lower voltage period 402, a writing pulse 403 and a second lower voltage period 404. The erasing pulse 401 and writing pulse 403 have substantially the same width and absolute value as the voltage but their polarities are opposite to each other. Similarly, two lower voltage periods 402 and 404 have substantially the same width but their polarities are opposite to each other. In this example, when the erasing pulse 401 is inputted, the output light turns off without regard to the existence of the inputted light. And when the writing pulse 403 is inputted, the output light turns on only when the inputted light is present. A typical example of the intensity of the output light 405 responding to the driving signals shown in the time-chart (a) in FIG. 4 is shown in time-chart (b) in FIG. 4.
Still another example of the driving signals of the spatial light modulator is shown in FIG. 5. In FIG. 5, time-chart (a) shows the wave form of the driving-signals, and time-chart (b) shows the intensity of the output light from the spatial light modulator. As shown in the time-chart (a) in FIG. 5, one driving signal consists of an erasing pulse 501, a first lower voltage period 502, a writing pulse 503 and a second lower voltage period 504. In the above-mentioned examples shown in FIGS. 3 and 4, the duty ratio of the output light is at most 504. On the other hand, as shown in FIG. 5, by changing the widths of the first and second lower voltage periods 502 and 504, the duty ratio of the intensity of output light 505 can be made nearly equal 1. In such a method, the mean value of the intensity of the output light becomes larger and the contrast thereof is also increased. The driving signal becomes unsymmetrical to the time base, but the deterioration of the switching characteristics can be prevented even though the DC component of the driving signal averages out to be zero. In addition, a conductive material is-used as the alignment film, the deterioration of the switching characteristics can be further prevented.
In case of using the spatial light modulator for the projection-type display apparatus or holographic television set, the brightness of the screen is necessary to be completely uniform. It is, however, difficult to make the brightness of each part of the spatial light modulator completely uniform when the driving cycle of the spatial light modulator is completely in synchronism with the frame cycle of the CRT which is used as a writing means. Such a phenomenon is described referring to FIG. 6. In FIG. 6, plan view (a) shows the scanning lines 601, 603 on a screen 605; time-chart (b) shows the wave form of the driving signal of the CRT and the spatial light modulator; time-chart (c) shows luminance intensity of a phosphor 602 on the screen 605; time-chart (d) shows the intensity of the output light from a pixel of the spatial light modulator corresponding to the phosphor 602; time-chart (e) shows luminance intensity of a phosphor 604 on the screen 606; and time-chart (f) shows the intensity of the output light from a pixel of the spatial light modulator corresponding to the phosphor 604. Generally, the screen 605 of CRT is scanned by electron beams, the phosphor 602, 604 which are irradiated by the electron beam output pulses of light, and thereby, an image is formed on the screen 605. In case of scanning the electron beams on the CRT screen 605 from the up side to the down side, the phosphor 602 which is positioned on an upper scanning line 601 outputs the light in an earlier time in the frame cycle of the CRT. The phosphor 604 which is positioned on a lower scanning line 602 outputs the light later than the phosphor 602. As a result, when the reading time period 607 is made substantially equal to one frame cycle time of the CRT, the light pulses from the phosphors 602 and 604 are respectively shown by the wave forms designated by numerals 608 and 610 in the time-charts (c) and (e) in FIG. 6. And the intensities of the output lights from the pixels of the spatial light modulator responding to the light pulses 608 and 610 respectively become the wave forms designated by numerals 609 and 611 in the time-charts (d) and (f) in FIG. 6. Even though the intensities of the light pulses 608 and 610 are substantially the same, the mean value of the intensity of the output light 611 becomes smaller than that of the output light 609 in the same frame cycle. Accordingly, the output light 611 is felt darker than the output light 609 by human eyes, and it is felt that the upper left portion of the screen 605 is brighter and the lower right portion is darker. For the above-mentioned reasons, the brightness of the output image from the spatial light modulator in each part becomes inhomogeneous.